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Eric Bruno

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Interesting Java Puzzler

June 11, 2009

Here's an interesting code puzzler I found on Alex Miller's technical blog:

<font color="#7f0055">import</font> java.util.HashMap;  
<font color="#7f0055">import</font> java.util.HashSet;  
<font color="#7f0055">import</font> java.util.Map;  
<font color="#7f0055">import</font> java.util.Set;  
<font color="#7f0055">import</font> java.util.concurrent.ConcurrentHashMap;  
<font color="#7f0055">public class</font> EntrySetSize {  
  <font color="#7f0055">public static void</font> main(String[] args) {  
    printSetSize(<font color="#7f0055">new</font> ConcurrentHashMap());  
    printSetSize(<font color="#7f0055">new</font> HashMap());  
  <font color="#7f0055">private static void</font> printSetSize(Map map) {  
    map.put("<font color="#2a00ff">hello</font>", "<font color="#2a00ff">world</font>");  
    map.put("<font color="#2a00ff">world</font>", "<font color="#2a00ff">hello</font>");  
    Set set = <font color="#7f0055">new</font> HashSet(map.entrySet());  
    System.out.print(set.size() + " ");  

 The puzzle is to figure out what sequence of numbers this code will print when executed. Is it:

a) 2  2

b) 1  2

c) 0  2

d) something else

To find the answer (hint: it varies on different versions of the JDK) check out Alex's  blog here.

 He also has an excellent description of the puzzler's behavior.


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